To get to Piazza Navona from Civitavecchia, I'd take the regional train to Roma Trastevere, and a taxi or the tram from there. The number 8 tram stops outside the station. You'd want the tram going in direction Piazza Venezia. You would get off at the second stop after crossing the river, and it's about a ten-minute walk from there.

Or you could follow the directions I gave above, and get off the train at the San Pietro station. The number 64 bus passes very near Piazza Navona. However, the bus gets very crowded after leaving the Vatican, and it's not easy to know where to get off. Many buses now have electronic boards announcing the next stop and all the things to see in the vicinity, but you can't guarantee you'll be on such a bus. The last time I was on a bus in Rome, they had the signs, but they weren't working, and just displayed an ad over and over.

Travestere station is small compared to Termini and trams right out front to like bvlenci says - much easier getting taxis too at a smaller station.

Here are some problematic issues with regional tickets bought online.

The four-hour time window begins at the time you requested. E.g., if you buy a regional ticket online for a 9:58 train, you can use it on that train or any other regional train up until 13:58. However, if you get to the station early, you can't use it on any earlier train. For this reason, I would suggest getting a ticket for one or two hours earlier than your expected arrival time.

Regional tickets can't be exchanged for a different type of ticket. I once bought several tickets for a trip from Senigallia to Milan, regional train as far as Bologna and a Frecciarossa (high-speed) from there to Milan. When I got to Senigallia, I saw that the regional train was running an hour late, which would make us miss the connection in Bologna. I asked to exchange our regional tickets, paying the difference, for an Intercity train arriving in half an hour, which would get me there in time, because it makes fewer stops. I couldn't exchange the regional tickets, because they were bought online. If I had bought them at a station, and hadn't stamped them yet, I could have done it. I just threw away the regional tickets and bought the IC tickets.

In this case, there shouldn't be a problem with the regional tickets, but I advise starting your four-hour window on the early side.

>>Well some Italian rail experts - not me - <<

Is most of what you've posted third hand?

Nope 2nd hand from bvlenci and others - got a problem with that?